Tire pressure sensor for motor vehicles

ABSTRACT

A tire pressure sensor to be fastened to a rim of a vehicle wheel for sensing an inflation pressure of a tire mounted on the rim, the tire pressure sensor comprising a reference pressure chamber having a gas filled reference pressure volume, an electrically conductive diaphragm for sealingly separating the reference pressure chamber at a tire interior, a sensor electric circuit, a contact pin located in the reference pressure chamber and cooperating with the electrically inductive diaphragm for closing the sensor electric circuit when the tire inflation pressure is at a predetermined value, and an additional pressure chamber communicating with the reference pressure chamber and deformable to change the reference pressure volume therein to thereby establish a predetermined reference pressure in the reference pressure chamber.

BACKGROUND OF THE INVENTION

The invention relates to a tire pressure sensor for motor vehicles.

It is known from DE-OS No. 28 32 447 to screw in a tire pressure sensorat the circumference of a wheel rim for monitoring the inflationpressure in the tire, which tire pressure sensor comprises a pressureswitch which is to be actuated by the tire pressure. The pressure switchcomprises, on one hand, an electrically conductive diaphragm whichcloses a reference pressure chamber in the direction of the tire and, onthe other hand, a contact pin which is sealed in a glass bushing in thereference pressure chamber so as to be electrically insulated. Themiddle area of the diaphragm contacts the contact pin so as to close anelectric circuit when there is sufficient inflation pressure in thetire. The reference pressure chamber is filled via a duct with air ornitrogen to a desired reference pressure and is then sealed so as to betight against pressure. The reference pressure is selected in such a waythat the diaphragm contacts the end of the contact pin when there issufficient inflation pressure in the tire. However, if the inflationpressure in the tire falls below a minimum pressure predetermined by thereference pressure the diaphragm is lifted from the contact pin andopens a resonant circuit which comprises a coil and a capacitor and isarranged in the tire pressure sensor. It can be determined whether ornot the pressure switch is closed by a signal receiver arranged in thevicinity of the wheel, every time the tire pressure sensor runs past atevery revolution of the wheel via a high-frequency oscillation of theevaluating circuit which is fixedly arranged in the vehicle.

Such known constructions have the disadvantage that the referencepressure chamber is filled with the gas during the manufacturing of thetire pressure sensor, the sensor is adjusted to a predeterminedreference pressure, and the reference pressure chamber is then welded soas to be gastight, so that it is no longer possible to subsequentlyadapt the reference pressure to different values which are predeterminedby the type of vehicle, by the tire, the speed or the loading of thevehicle. Therefore, a quantity of pressure sensors with different tirepressures must be produced and kept in storage.

In another solution known from DE-PS No. 33 10 052, the referencepressure in the reference pressure chamber of the tire pressure sensoris adjustable in a variable manner in that the tire pressure is fixedsimultaneously as new reference pressure for the reference pressurechamber via a valve arrangement during the tire filling process.However, this has the disadvantage that the valve arrangement does notensure the sealing of the reference pressure chamber necessary for thelife of the tire pressure sensor so that, in the event of a leaky valve,air escapes from the reference pressure chamber when a difference inpressure occurs between the reference pressure chamber and the tire,which defect is not noticed. An insufficient tire pressure during agradual drop in pressure in the tire is then no longer indicated.

SUMMARY OF THE INVENTION

The object of the present invention is to develop a tire pressure sensorwith a reference volume which is sealed in a gastight manner in such away that it can be adjusted in a fixed manner to different referencepressures. The object of the invention is achieved by a tire pressuresensor in which the entire gas filled reference volume can be changed toadjust to a desired reference pressure.

The design of the tire pressure sensor, according to the invention, hasthe advantage that only one single construction of the tire pressuresensor is necessary for the different desired reference pressures, sothat expenditure on manufacturing and storage can be considerablyreduced. The pressure in the reference pressure chamber can now becalibrated to the desired value relating to the specific vehicle priorto the installation of the reference pressure sensor at the vehiclewheel. A further advantage consists in that the filling opening isclosed so as to be tight against pressure after the reference pressurechamber is filled with gas, so that the subsequently adjusted referencepressure is retained unchanged thereafter.

It is particularly advantageous that the reference pressure chambercommunicates with an additional sealed pressure chamber arranged in thehousing of the tire pressure sensor via at least one opening; the volumeof the additional pressure chamber can be changed by means ofdeformation for the purpose of adjusting the desired reference pressure.In addition, it is possible, in principle, to adjust the desiredreference pressure starting from a low pressure in the reference chamberby contracting the additional pressure chamber, as well as to achievethis reference pressure starting from a higher pressure by expanding theadditional pressure chamber. It is particularly advisable that theadditional pressure chamber be arranged on the front side of thereference pressure chamber facing away from the diaphragm and that itconsist of a bellows which can be contracted in the axial direction ofthe reference pressure chamber. In an advantageous manner, the bellowscontacts the front side of the reference pressure chamber provided withthe opening with one end and contacts a closing plate with the otherend, wherein the reference pressure chamber and closing plate areaxially displaceable relative to one another in the housing of the tirepressure sensor.

The present invention as to its construction so to its mode ofoperation, together with additional objects and advantages thereof, willbe best understood from the following detailed description of thepreferred embodiments with reference to accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a partial cross-sectional view of a vehicle wheel with atire pressure sensor according to the invention, which is inserted inthe wheel rim; FIG. 2 shows a cross-view of a partial cross-sectionalview of first embodiment of the tire pressure sensor, according to theinvention, in enlarged scale; FIG. 3 shows a second embodiment of thetire pressure sensor according to the invention, in enlarged scale; FIG.4 shows a partial cross-sectional view of yet another embodiment of thetire pressure sensor according to the invention with an additionalpressure chamber; and FIG. 5 shows a partial cross-sectional view of afurther embodiment of the tire pressure sensor according to theinvention with an adjusting screw for the reference pressure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a portion of a motor vehicle wheel is shown in a reducedscale in cross section and designated by 10. It shows a tire 11 which ismounted on a wheel rim 12. A tire pressure sensor 13 is screwed into athreaded hole 14 of the wheel rim 12 at an inwardly directed portion ofthe wheel rim 12. At a slight distance from the latter, a signalreceiver 15 is fastened in the area of the tire pressure sensor 13 atthe wheel suspension of the vehicle, not shown, the front side of thesignal receiver 15 being directed toward the tire pressure sensor 13,and senses a high-frequency oscillation in the tire pressure sensor 13when the latter passes by during every revolution of the vehicle wheel10.

The construction of such a tire pressure sensor 13 is shown in FIG. 2.It consists of a sensor housing 16 of noble metal which comprises athreaded shoulder 17 for fastening at the wheel rim 12. The sensorhousing 16 is open in the front in the direction of the tire 11. Anannular shoulder 18 is located on the inside in the middle area of thesensor housing 16. A reference pressure chamber 19 made of steelcomprising a collar 20 which is directed radially outward is insertedinto the sensor housing 16 from the front and is supported at theannular shoulder 18 of the sensor housing 16 via a sealing ring 21. Thereference pressure chamber 19 carries a spacer ring 22 of fibrousplastics material at its collar 20. Above this, a perforated cover disk23 is inserted into the opening of the sensor housing 16, and theseparts are clamped in the sensor housing 16 by a flanged rim 24 at thefront side of the sensor housing 16. The reference pressure chamber 19is closed in the direction of the tire interior 11 by a flat steeldiaphragm 25. In the center of the reference pressure chamber 19, acontact pin 26 with a glass bushing 27 is fastened at the base 28 of thereference pressure chamber so as to be tight against pressure andelectrically insulated. At its front end, the contact pin 26 carries acontact tip 29 which, together with the diaphragm 25, forms a pressureswitch which is actuated as a function of the pressure differential onthe two sides of the diaphragm 25.

A resonant circuit chamber 30 is located in the lower area of the sensorhousing 16, a toroidal coil 31 on a ferrite core 32 which is connectedto a series resonant circuit, and a capacitor, not shown, are arrangedin the resonant circuit chamber 30. These parts are received in a covercap 33 consisting of plastic.

An additional pressure chamber 34, which communicates with the pressurespace 36 of the reference pressure chamber 19 via two openings 35 in thebase 28 of the reference pressure chamber 19, is arranged between thereference pressure chamber 19 and the resonant circuit chamber 30 on thefront side of the reference pressure chamber 19 facing away from thediaphragm 25. This additional pressure chamber 34 comprises a bellows 37which can be contracted in the axial direction of the reference pressurechamber 19. The bellows 37 contacts the front side of the referencepressure chamber 19 with its upper end and contacts the closing plate 38with its other end so as to be gastight, wherein the openings 35 in thebase 28 of the reference pressure chamber 19 are enclosed by the bellows37. The pressure space 36 of the reference pressure chamber 19constantly communicates with the pressure space 39 of the additionalpressure chamber 34 via the openings 35. They form a reference volumewhich is filled with a reference gas, e.g. nitrogen, prior to theinsertion of the reference pressure chamber 19 with pressure chamber 34into the sensor housing 16 via a filling opening 40 in the closing plate38. The pressure of the gas in the reference volume is selected in sucha way that it lies below the subsequently desired reference pressure.The filling opening 40 is then closed so as to be tight against pressureby a steel ball 41 which is pressed into the filling opening 40 andwelded. The contact pin 26 is electrically connected with a connectionpin 43 by its lower end via a helical spring 42, the connection pin 43being fastened in a glass bushing 44 in the closing plate 38 so as to beelectrically insulated and tight against pressure. The free end of thisconnection pin 43 is connected with the resonant circuit in the resonantcircuit chamber 30 via a flexible wire 43a at a connection plate 45.

In order to be able to adjust the desired reference pressure in thereference pressure chamber 19, it is provided to change the referencevolume by deforming the additional pressure chamber 34 in that thebellows 37 is pressed together axially. For this purpose, a clampingring 46 is provided which carries the closing plate 38 of the additionalpressure chamber 34 with a support 47, which projects inward in aring-shaped manner, and is inserted, preferably pressed into the sensorhousing 16 from below. In addition, the cover cap 33 is sealed relativeto the clamping ring 46 by a sealing ring 48 and is clamped by a flangedrim 49 of the clamping ring 46. A carrying member 50 consisting ofplastic for the ferrite core 32 and the toroidal coil 31 is supported atthe back of the closing plate 38.

During the production of the tire pressure sensor 13, the clamping ring46 is pressed approximately halfway into the sensor housing 16 after thepressure spaces 36, 39 of the reference pressure chamber 19 and theadditional pressure chamber 34 are filled with the gas and after thefilling opening 40 is sealed. The desired reference pressure must bedetermined with regard to the specific vehicle prior to the finalinstallation of the tire pressure sensor 13 in the wheel rim 12 and thisreference pressure is now applied to the diaphragm 25 from the outside,e.g. in a pressure device suitable for this purpose. The clamping ring46 is now pressed into the sensor housing 16 from the rear by acorresponding tool until the reference pressure is increased enough sothat the diaphragm 25 is lifted from the contact tip 29 by reducing thereference volume by the closing plate 38 which is axially displaceablerelative to the reference pressure chamber 19 via the annular support47. At this point in time, the pressure differential on the two sides ofthe diaphragm 25 is equal to zero, i.e. the desired reference pressureis now adjusted in the reference pressure chamber 19. The lifting of thediaphragm 25 from the contact tip 29 can be detected by means of anoscillation which is coupled at the toroidal coil 31 of the resonantcircuit chamber 30. The clamping ring 46 is now fixed in this positionby a counter-nut 5 at the sensor housing 16. A permanent deformation ofthe bellows 37 and change in the reference volume is accordinglyeffected for the purpose of calibrating the tire pressure sensor 13 tothe desired reference pressure.

In the construction according to FIG. 2, the desired reference pressurecan also be adjusted from the outside after inserting the tire pressuresensor 13 at the wheel rim 12 in that the desired reference pressure isfirst produced in the tire, and the reference volume is then reduced bythe axial contraction of the additional pressure chamber 34 via theclamping ring 46 until the diaphragm 25 is lifted from the contact pin26 at the desired reference pressure. If necessary, the clamping ringcan also be screwed into the sensor housing 16 by a thread in order toadjust the reference pressure and subsequently secured. In such aconstruction, it is also possible to change the reference pressure at alater time in that the securing means is first removed in order to carryout a new calibration of the reference pressure by screwing orunscrewing the clamping ring 46 at the sensor housing 16.

The adjustment of the desired reference pressure can be realized indifferent ways to the extent that the reference pressure chamber 19 andthe closing plate 38 of the additional pressure chamber 34 in the sensorhousing are axially displaceable relative to one another by a clampingring or the like. Accordingly, the construction can also be selected insuch a way that the clamping ring is inserted at the opening of thesensor housing 16 directed toward the tire.

The embodiment according to FIG. 3 shows such an arrangement, whereinonly the right half of the tire pressure sensor 60 is shown in crosssection in enlarged scale because of the symmetrical construction. Inthis instance, a reference pressure chamber 61 is received in a clampingring 63 by its outwardly directed collar 62, which clamping ring 63holds the cover disk 23 in the front by a flanged rim 64 for protectingthe diaphragm 25 and is inserted in the sensor housing 65 from above. Anannular additional pressure chamber 66 is arranged at the back side ofthe reference pressure chamber 61 and consists of a bellows 67 which isarranged annularly around the contact pin 69 projecting out of thereference pressure chamber 61 so as to be tight against pressure, thebellows 67 being supported against the reference pressure chamber 61 onthe one side and against a correspondingly annular closing plate 68 onthe other side. Accordingly, it is possible to guide the contact pin 69of the reference pressure chamber 61 in one piece up to the resonantcircuit chamber 30 via the glass bushing 27. Moreover, the fillingopening 70 for the reference gas in the base 28 of the referencepressure chamber 61 can be arranged directly next to the glass bushing27 without being covered by the annular pressure chamber 66. Thereference pressure chamber 61 and annular pressure chamber 66 alsocommunicate with one another in this instance via the openings 35 in thebase 28 of the reference pressure chamber 61. The annular closing plate68 lies on an annular shoulder 71 of the sensor housing 65 which extendsvery far inward. The resonant circuit chamber 30, with its toroidal coil31, the ferrite core 32, the carrying member 50 and the capacitor, notshown, lie below the latter. The cover cap 33 is grasped by a flangedrim 72 of the sensor housing 65.

During the production of the tire pressure sensor 60, a prefabricatedunit comprising the clamping ring 63, reference pressure chamber 61,cover disk 23 and annular pressure chamber 66, is first inserted fromthe top into the opening of the sensor housing 65, wherein the clampingring 63 is inserted into the sensor housing 65 approximately halfway.The reference pressure chamber 61 and the annular pressure chamber 66are filled with gas via the filling opening 70 to a low limiting valueof the various reference pressures, and the filling opening 70 is thenclosed so as to be tight against pressure by with the ball 41. Next, theresonant circuit chamber 30 is arranged at the back side of the annularshoulder 71 and fastened there by the flanged rim 72 of the sensorhousing 65. Prior to the installation of the tire pressure sensor 60,the reference pressure can now be increased to the desired value by thecontinued pressing in of the clamping ring 63 by the reference pressurechamber 61 which is axially displaceable with the clamping ring 63relative to the cover plate 38, which cover plate 38 is supported in thesensor housing 65 so as to be fixed.

In this instance, also, the final position of the clamping ring 63 canbe fixed by a securing ring 51 at the sensor housing 65. The clampingring 63 can also be screwed into the sensor housing 65 by means of athread. In this case, a counter-nut can be used instead of the securingring 51.

FIG. 4 shows the basic construction of another embodiment of a tirepressure sensor 80 which is provided with the same reference numbers tothe extent that it conforms to the embodiment according to FIG. 2. Thus,in FIG. 4, the reference pressure chamber 19 is fastened in the upperportion of the sensor housing 81 in the same way as in FIG. 2. However,in this instance, the additional pressure chamber 34 is not fastened ata clamping ring via the closing plate 38 as in FIG. 2; rather, it isheld by a catch lock 82 which is supported in the sensor housing 81 soas to be swivelable and rests against the outer rim of the closing plate38 by the force of a pressure spring 83 supported at the sensor housing81. The closing plate 38 is secured against axial displacement by thecatch lock 82. The catch lock 82 has a downwardly directed extension 85arranged below its bearing 84 in a housing opening 86. By means ofinserting a screw driver in this housing opening 86, the catch lock 82can be unlocked by acting on the extension 85 so that the closing plate38 of the additional pressure chamber 34 is axially displaceable inorder to adjust a desired reference pressure in the reference pressurechamber 19.

For this purpose, an additional pressure chamber 87, which is to befilled with compressed air via an opening 88 in the form of a connectionnipple which leads outward, is arranged at the back side of the closingplate 38. This additional pressure chamber 87 likewise consists of abellows 89 which is supported at the top against the back side of theclosing plate 38 and is closed at the bottom by a base plate 90 whichcarries the opening 88. The base plate 90 is fastened at the sensorhousing 81 and is supported against it at the bottom, respectively.

If this additional pressure chamber 87 is now filled with compressed airvia the opening 88, the closing plate 38 is pressed up in the axialdirection of the tire pressure sensor 30 by this pressure when the catchlock 82 is unlocked. The reference volume of the pressure spaces 36 and39 is accordingly reduced and the gas which is introduced into it priorto assembly is compressed. The desired reference pressure can beadjusted by lifting of the closing plate 38, as described in FIG. 2, inthat the additional pressure chamber 87 is now filled with the samepressure. The catch lock 82 is then caught again at the rim of theclosing plate 38, and the closing plate 38 is accordingly securedagainst axial displacement. The compressed air can now escape again fromthe additional pressure chamber 87 without the reference pressure beingchanged accordingly.

In an advantageous manner, the compressed air connection at theadditional pressure chamber 87 can be constructed in such a way that itis accessible from the outside and can be filled with compressed air bymeans of a tire filling device. If the reference pressure no longerchanges subsequently, the housing opening 86 is to be securely closedand sealed, respectively, after the calibration of the referencepressure.

Another basic possibility for solving the problem suggested by theinvention is shown in FIG. 5. It shows the reference pressure chamber 19in the area of the base opening 35. A corrugated diaphragm 91, whichencloses the opening 35 and constitutes an additional pressure chamberwith variable volume, is provided at the back side of the referencepressure chamber 19. The corrugated diaphragm 91 is contracted via aclosing plate 94 by means of an adjusting screw 93 which can be screwedinto the sensor housing 92, and the pressure in the reference pressurechamber 19 is accordingly adjusted to the desired value. The adjustingscrew 93 is then advisably sealed so that the adjusted referencepressure cannot be inadvertently changed.

The solution according to the present invention is not limited to theshown embodiments since alternative design solutions are possible withinthe framework of the invention. However, it is essential for theadjustment of the desired reference pressure in the reference pressurechamber of the tire pressure sensor to change the reference volumefilled with the gas and to fix the reference volume when the desiredreference pressure is reached. This can be achieved, according to theinvention by providing additional pressure chamber with variable volume,but can also be achieved by a changeable volume in the referencepressure chamber itself.

While the invention has been illustrated and described as embodied in atire pressure sensor for motor vehicles, it is not intended to belimited to the details shown, since various modifications and structuralchanges may be made without departing in any way from the spirit of thepresent invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:
 1. A tire pressure sensor to befastened to a rim of a vehicle wheel for sensing an inflation pressureof a tire mounted on the rim, said pressure tire sensor comprisingreference pressure chamber means having a filled reference pressurevolume and including a reference pressure chamber for monitoring thetire inflation pressure; an electrically conductive diaphragm sealinglyseparating said reference pressure chamber from a tire interior; asensor electric circuit is said sensor; a contact pin located in saidreference pressure chamber and removably contacting said electricallyconductive diaphragm for closing said sensor electric circuit when thetire inflation pressure is at a predetermined value; and means defininga deformable portion of the reference pressure chamber means andenclosing at least partially said reference chamber volume, and meansother than the tire pressure for deforming said deformable portion tothereby change said reference chamber volume and establish apredetermined reference pressure in said reference pressure chamber. 2.A tire pressure sensor according to claim 1, further comprising ahousing; said reference pressure chamber means being located in saidhousing and further including an additional pressure chamber and anopening for communicating said additional pressure chamber with saidreference pressure chamber, said additional pressure chamber definingsaid deformable portion of said reference pressure chamber means.
 3. Atire pressure sensor according to claim 2, wherein said referencepressure chamber has an axis, said additional pressure chamber beingarranged on an end of said reference pressure chamber opposite to saiddiaphragm facing end of said reference pressure chamber and including abellows contractible in an axial direction of said reference pressurechamber.
 4. A tire pressure sensor according to claim 3, wherein saidreference pressure chamber means includes a plate means defining saidopposite end of said reference pressure chamber, said opening beingformed in said plate means, and a second plate axially displaceablerelative to said plate means, said bellows extending between said platemeans and said second plate.
 5. A tire pressure sensor according toclaim 4 further comprising a clamping ring at least partially located insaid housing for axially displacing said second plate relative to saidplate means.
 6. A tire pressure sensor according to claim 5, whereinsaid reference pressure chamber is fixed in said housing, said clampingring having an annular support for displacing said second plate axiallyrelative to said plate means.
 7. A tire pressure sensor according toclaim 5, wherein said second plate is fixed in said housing, saidclamping ring axially displacing said plate means relative to saidsecond plate.
 8. A tire pressure sensor according to claim 4, whereinsaid contact pin projects out of said reference pressure chamber meansand extends beyond said second plate, said opening means comprising twoopenings located on opposite sides of said contact pin, said bellowsmeans comprising two bellows surrounding said openings, respectively. 9.A tire pressure sensor according to claim 4, wherein said second platecomprises a gas filling opening, said tire pressure sensor furthercomprising means for gas-tight closing said filling opening.
 10. A tirepressure sensor according to claim 4, wherein said second plate has afront end surface closing said additional pressure chamber and a backend surface, said reference pressure chamber means comprising a secondadditional chamber located on the back end surface of said second plateand having an opening for filling said second additional pressurechamber with gas, said second plate being displaceable under an actionof pressure in said second additional pressure chamber.
 11. A tirepressure sensor according to claim 10, further comprising an unlockablecatch lock located in said housing for securing said second plateagainst axial displacement.
 12. A tire pressure sensor according toclaim 1, wherein said reference pressure chamber means comprises acorrugated diaphragm defining an additional pressure chamber and anopening for communicating said additional pressure chamber with saidreference pressure chamber, said pressure sensor further comprisingscrew means for changing a volume of said additional pressure chamber tothereby establish a predetermined reference pressure in said referencepressure chamber.